1. Field of the Invention
The present invention relates to high temperature, erosion resistant coatings, and more particularly relates to the use of such coatings as abradable seals and thermal barrier coatings.
2. Background Information
Most components of combustion turbines are operated at very high temperatures and often require the use of specialized coatings/inserts to protect underlying supporting materials. These specialized coating/inserts include thermal barrier coatings (TBCs), disposed opposite the turbine blade tips, as taught in U.S. Pat. No. 5,180,285 (Lau).
Conventional TBCs typically comprise a thin layer of zirconia. In many applications, the coatings must be erosion resistant and must also be abradable. For example, turbine ring seal segments, which fit with tight tolerances against the tips of turbine blades, must withstand erosion and must also preferentially wear or abrade in order to reduce damage to the turbine blades, and form a tight seal with the turbine blade. Protective coating systems can include several layers including a metallic bond or barrier coating of MCrAlY having an alumina scale and, for example, a columnar yttria stabilized zirconia thermal barrier, as taught in U.S. Pat. No. 4,916,022 (Solfest et al.), which can be further coated by an erosion resistant layer of alumina or silicon carbide, as taught by U.S. Pat. No. 5,683,825 (Bruce et al.).
In U.S. Pat. No. 5,780,146 (Mason et al.), 30 wt. % to 50 wt. % (50 vol. % to 60 vol. %) of hollow alumino silicate or alumina spheres of 400 micrometer to 1800 micrometer diameter, and having a high temperature capability of approximately 1300xc2x0 C., were used in an aluminum phosphate matrix, for an abradable seal. The seal is used over a ceramic matrix composite shroud segment, which may comprise silicon carbide fibers in an alumina matrix. However, this invention is limited in thermal stability due to uncontrolled sphere distribution and contact, therefore, the matrix controls the thermal stability of system and limits the temperature of the system to less than 1200xc2x0 C.
Fillers have also been used by Naik et al., in U.S. Pat. No. 5,064,727. There, abradable stationary seal walls, for jet turbine housings which seal opposing, rotating rotor blade tips, have a ceramic core containing from 30 vol. % to 98 vol. % solid ceramic filler, where the ceramic fills a honeycomb wall structure. This is then covered with erosion and corrosion resistant outer layer, which is made porous by uniformly dispersed, finely divided filler. The pores can be filled with ceramic, metal oxide or carbide materials. Fillers mentioned include hollow ZrO2.8YO3 ceramic spheres and solid Al2O3, SiC, TiC and BN spheres.
Other abradable honeycomb structures for use in turbines are taught in U.S. Pat. No. 4,867,639 (Strangman). There, low melting fluorides, such as BaF2, are incorporated into a stabilized zirconia or alumina matrix which, in turn, is used to fill a honeycomb shroud lining made of, for example, a metal alloy. The filling becomes molten when the rotating blade tips rub the shroud, and upon resolidification, improve the smoothness of the abraded surface. Ainsworth et al., in U.S. Pat. No. 4,639,388, teaches another variation of reinforced ceramic layers, including a honeycomb matrix for use in a turbine as abradable seals.
In U.S. patent application Ser. No. 09/261,721, Merrill et al., filed on Mar. 3, 1999), a honeycomb structure having open cells was filled, and optionally overlaid, with a material containing hollow ceramic particles embedded in an interconnected ceramic matrix, to provide a composite thermal barrier composite coating having superior erosion resistance and abrasion properties for use on combustion turbine components. The hollow particles were preferably spherical and made of zirconia, alumina, mullite, ceria, YAG or the like, having an average particle size of about 200 micrometers (0.2 mm) to 1500 micrometers (1.5 mm). The steady state erosion rate, grams lost/kg erosive impacting media, of this filler was 3.2 g/kg vs. 4.6 to 8.6 g/kg for conventional TBCs. Here, the ceramic matrix comprised an interconnected open cell honeycomb structure, binding the hollow spheres together where the hollow spheres were bonded by a network of aluminum phosphate bridging bonds.
In U.S. patent application Ser. No. 09/536,742, filed on Mar. 28, 2000, a vacuum packing/impregnation method of bonding hollow geometric shapes was described, to provide abradable, thermally stable seals and the like. Both U.S. patent application Ser. No. 09/049,369, Morrison et al., filed on Mar. 27, 1998, now U.S. Pat. No. 6,197,424) and Ser. No. 09/049,328, Merrill, filed on Mar. 27, 1998, now U.S. Pat. No. 6,013,592), teach ceramic insulating coatings with improved erosion resistance and macroscopic closed porosity, utilizing hollow oxide-based spheres which can contact at least 3 or 4 other hollow spheres to provide improved dimensional stability at temperatures up to about 1600xc2x0 C. Erosion rate, grams lost/kg erosive impacting media was 4.5 g/kg and 7.5 g/kg.
However, none of these coatings or seal structures have optimized abradability with erosion resistance and insulating capability, minimized shrinkability and thermal mismatch, provided constrained stabilized uniform spherical porosity and adequate flexibility, and optimized thermal stability for operation substantially up to 1600xc2x0 C.; all of which characteristics will be required of the next generation high temperature turbine TBCs, seals and the like, as well as in non-turbine coating applications. What is needed are high temperature coatings, and composites that fill these and other future requirements.
Also, thermally sprayed structures having hollow spheres co-sprayed to introduce porosity for either abradability or reduced thermal conductivity, are limited to small sphere sizes, typically less than 200 microns, for spraying capability. These small spheres tend to melt in plasma and result in non-spherical pores which are not thermally stable. Such small scale porosity leads to poor erosion resistance. Additionally, thermally sprayed coatings/structures for abradable seals based on co-spray of fugitive particles, for example, polyester resin particles, which are subsequently burned out to leave increased porosity, results in small, non-spherical porosity and matrix-dominated properties which limit thermal stability. The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.
Therefore, it is one of the main objects of this invention to provide a high temperature, erosion resistant coating and material which is bondable, generally non-shrinking, abradable, flexible, thermally stable up to at least 1600xc2x0 C., and which has constrained stabilized porosity and insulating properties, as well as controlled thermal conductivity and thermal expansion properties.
These and other objects of the invention are accomplished by providing a material system, useful as an erosion resistant high temperature layer, comprising a substantially close packed array of generally contacting, hollow, individually formed geometric shapes, having a coordination number greater than or equal to 1 and having a 70% to 100% dense wall structure, which are bonded together, and which material system has a constrained stabilized porosity and is abradable, thermally insulating, thermally stable and substantially non-shrinking at temperatures up to at least 1600xc2x0 C. Wall thickness greater than about 100 micrometers is preferred, in order to provide good erosion resistance. This material provides an optimized combination of physical and thermal properties needed in the industry but heretofore not attainable, but which will be essential in the future.
Preferably, the geometric shapes are selected from rigid, hollow, essentially closed ceramic spheres and other similar geometric shapes of low aspect ratio, less than 10 and preferably less than 5, such as hollow cylinders and the like. The shapes are xe2x80x9cindividually formed,xe2x80x9d defined here as meaning formed separately and then; stabilized during manufacture, rather than being formed in situ on a substrate etc. The hollow ceramic shapes have xe2x80x9cdensexe2x80x9d walls, defined here As having a density from 70% to 100% of theoretical (0% to 30% porous). Because the geometric shapes are independently formed, denser wall formation results, which allows crack deflection and general toughening of the material, as well as allowing geometric stability to very high temperatures approaching 1700xc2x0 C. Hollow spaces between, for example, one diameter of a first large geometric shapes can be filled with second, smaller diameter geometric shapes, to reduce void volume and minimize, consistent with some measure of flexibility, the content of matrix ceramic bonds which help bond the shapes together.
Preferably, there are three dimensional xe2x80x9cchainsxe2x80x9d of hollow shapes, where a substantial number of shapes contact at least 4 to 12 preferably 6 to 10 other shapes. This chain or string-like geometry provides strength and minimizes large void volumes being close to each other. The material system should have some measure of porousness, at least 15 vol. % usually up to a maximum 90 vol. %, preferably 40% vol. to 70% vol. for turbine thermal insulating and abradable coatings, and preferably has some randomness of the contacting shapes in its structure. The material system of this invention due to its structure, is also highly friable, while maintaining a low elastic modulus.